I don't understand how to use this app

I encourage you to ask specific questions if you're having trouble.

A good guide for the app can be found by opening the Menu (found by hitting the three-line icon in the upper left hand corner of the app's main screens) then tapping "Help" and "Frequently Asked Questions." On the web, you can click the "Help" section located at the top right of the page.

You may also want to visit:

https://blog.myfitnesspal.com/essential-guide-to-myfitnesspal/